1. Field of the Invention
This invention relates to new transition metal-based supported olefin polymerization catalyst systems, novel methods of producing such catalysts and methods of polymerizing alpha-olefins to provide polyolefins, and preferably high density polyethylene and linear low density polyethylene. More particularly, this invention relates to the preparation of ultra high active catalyst compositions comprising at least a transition metal compound, a magnesium-containing compound and a polymeric material.
2. Description of the Prior Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
The field of olefin polymerization catalysis has witnessed many remarkable discoveries during the last 50 years. In particular, two broad areas of invention stand out. Firstly, the discovery of Ziegler-Natta catalysts in the 1950""s, which are still being used extensively in the polyolefins industry. Secondly, and more recently, the discovery of the highly active metallocene-based catalysts. Since the discoveries of these systems, extensive research work was conducted in order to improve their performance.
However, despite the progress in these areas, there are still certain limitations as recognized by those of ordinary skill in the art. For example, conventional Ziegler-Natta catalysts often display limited activity, which reflects on the high catalyst residues. On the other hand, the metallocene-based catalysts intrinsically possess high activity, though the catalyst precursors and, in particular, the cocatalysts required for polymerization, such as aluminoxanes or borane compounds, are very expensive. Further, another limitation that both catalyst systems share is the lengthy method of preparation.
Traditionally, the active components of both Ziegler-Natta and metallocene catalysts are supported on the inert carriers to enhance the catalyst productivity and improve and control the product morphology. Magnesium chloride and silica have predominantly been used for the preparation of supported olefin polymerization catalysts. U.S. Pat. No. 4,173,547 to Graff describes a supported catalyst prepared by treating a support, for example silica, with both an organoaluminum and an organomagnesium compound. The treated support was then contacted with a tetravalent titanium compound. In a simpler method, U.S. Pat. No. 3,787,384 to Stevens et al. discloses a catalyst prepared by first reacting a silica support with a Grignard reagent and then combining the mixture with a tetravalent titanium compound.
However, procedures typically used for the preparation of suitable magnesium chloride and silica supports such as spray drying or re-crystallization processes are complicated and expensive.
Hence, all methods described in the aforementioned patents of catalyst preparation present the inconvenience of being complicated, expensive and do not allow consistency of particle size and particle size distribution. Also, despite the extensive and increasing use of the described supports for Ziegler-Natta catalysts, the support materials themselves have several deficiencies. For example, in the case of silica, high calcination temperatures are required to remove water, which is a common catalyst poison. This represents a significant proportion of the preparation of the catalyst. The use of silica as a support results in the support remaining largely in the product, which can affect the product properties, such as optical properties, or processing.
Certain polymeric materials have also been used for supporting titanium and magnesium compounds. However, most of the polymeric supports used so far have been based on polystyrene or styrene-divinylbenzene copolymers. U.S. Pat. No. 5,118,648 to Furtek and Gunesin describe a catalyst prepared using styrene-divinylbenzene as a polymeric support. The preparation of the catalyst was carried out by suspending the polymeric support in a solution of a magnesium dihalide or a magnesium compound capable of being transformed into a magnesium dihalide, for example, by titanium tetrachloride treatment, and subsequently evaporating the solvent. Hence, the active catalyst components were deposited on the polymeric support by physical impregnation. Other physical impregnation methods include that described by U.S. Pat. No. 4,568,730 to Graves whereby polymer resins of styrene-divinylbenzene are partially softened and the active catalyst components homogeneously mixed in the resin to form a mass, which was subsequently pelletized or extruded into catalyst particles. However, the activity of the above-described polymer supported catalysts is not significantly higher than that of silica-based Ziegler-Natta catalysts.
Polypropylene and polyethylene have also found use as polymeric supports where the polymeric material is typically ground with the catalyst components, which represents a difficult and complicated catalyst preparation procedure. In addition, there remains a significant concern as to the ability of the support material to retain the active species, deposited by physical impregnation, during polymerization conditions and thus generate, for example, fines. Hsu et al., Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 32, 2135 (1994), has used poly(ethylene-co-acrylic acid) as a support for Ziegler-Natta catalysts. Though, the catalyst activity was found to be similar to that of the magnesium chloride supported catalyst.
The present invention provides ultra highly active supported olefin polymerization catalysts comprising at least one transition metal compound, at least one magnesium compound and defined polymer particles. The polymer particles used in catalyst preparation have a mean particle diameter of 5 to 1000 xcexcm and a pore volume of at least 0.1 cm3/g and a pore diameter of at least from 20 to 10,000 angstroms, preferably from 500 xc3x85 to 10,000 xc3x85 and a surface area of from 0.1 m2/gm to 100 m2/gm, preferably from 0.2 m2/gm to 15 m2/gm.
One aspect of the invention relates to improved catalyst systems. The solid catalyst component (catalyst precursor) used in the present invention contains at least a transition metal compound, at least a magnesium compound, and a polymeric material having a mean particle diameter of 5 to 1000 xcexcm, a pore volume of 0.1 cm3/g or above and a pore diameter of 20 to 10,000 angstroms, preferably from 500 xc3x85 to 10,000A and a surface area of from 0.1 m2/gm to 100 m2/gm, preferably from 0.2 m2/gm to 15 m2/gm.
The transition metal compound used for the synthesis of the solid catalyst component in the invention is represented by the general formula M(OR1)nX4xe2x88x92n, wherein M represents a transition metal of Group 4, 5, 6, 7 or 8-10 of the Periodic Table of the elements, R1 represents a hydrocarbon having 1 to 20 carbon atoms, X represents a halogen atom and n represents a number satisfying 0xe2x89xa6nxe2x89xa64. Nonlimiting examples of the transition metal are titanium, vanadium, or zirconium. Examples of R1 include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl and the like.
Preferred examples of the above mentioned compounds include the following: titanium tetrachloride, methoxy titanium trichloride, dimethoxy titanium dichloride, ethoxy titanium trichloride, diethoxy titanium dichloride, propoxy titanium trichloride, dipropoxy titanium dichloride, butoxy titanium trichloride, butoxy titanium dichloride, vanadium trichloride, vanadium tetrachloride, vanadium oxytrichloride, and zirconium tetrachloride.
The magnesium compounds used for the catalyst synthesis in the invention include Grignard compounds represented by the general formula R2MgX, wherein R2 is a hydrocarbon group of 1 to 20 carbon atoms and X is a halogen atom. Other preferred magnesium compounds are represented by the general formula R3R4Mg, wherein R3 and R4 are each a hydrocarbon group of 1 to 20 carbon atoms.
Preferred examples of the above mentioned compounds include the following: diethylmagnesium, dibutylmagnesium, butylethylmagnesium, dihexylmagnesium, ethylmagnesium chloride, butylmagnesium chloride, hexylmagnesium chloride and the like.
These magnesium compounds described above may also be used in catalyst preparation as a mixture with an organoaluminum compound. Examples of the organoaluminum compounds include trialkylaluminum such as trimethylaluminium, triethylaluminum, triisobutylaluminum, trihexylaluminum and the like; and alkylalumoxanes such as methylalumoxane, ethylalumoxane and the like. The mixture of the magnesium compound and the organoaluminum compound in this invention can be used with a Mg:Al molar ratio of 99:1 to 50:50, and preferably 98:2 to 80:20 and more preferably 96:4 to 85:15.
The polymer particles used in the present invention are in the form of distinct spherical particles, on which the active catalyst component is chemically bonded, wherein the ratio of active catalyst component to polymeric support is less than 1% by weight, preferably less than 0.7% by weight. In contrast, catalysts prepared in the prior art using polymeric materials relied on physical impregnation of the catalyst active sites on the polymeric materials.
The polymer particles used in the present invention have a spherical shape with a particle diameter of 5 to 800 xcexcm, preferably 10 to 600 xcexcm, and more preferably 15 to 500 xcexcm, a pore diameter of 20 to 10,000 angstroms preferably from 500 xc3x85 to 10,000 xc3x85, a surface area of from 0.1 m2/gm to 100 m2/gm, preferably from 0.2 m2/gm to 15 m2/gm and a pore volume of 0.1 cm3/g or above, preferably 0.2 cm3/g or above. Uniformity of particle size is not critical and in fact catalyst supports having nonuniform particle sizes are preferred. By way of example and not limitation, for a catalyst support having a median particle size of 65 microns, it is preferred that at least 10% of the support particles have a diameter of greater than 85 microns, and at least 10% of the support particles have a diameter of less than 45 microns.
Examples of the polymeric particles used as supports in the catalyst preparation of the present invention include thermoplastic polymers. Polymer particles of polyvinyl chloride are preferred, and non-crosslinked polyvinyl chloride particles are most preferred.
The polymer particles used in the present invention have surface active sites such as labile chlorine atoms. Preferably, these active sites are reacted stoichiometrically with the organometallic compound, namely a magnesium and/or aluminum containing compound.
The use of the polymer particles mentioned in this invention in catalyst preparation offers significant advantages over traditional olefin polymerization catalysts using supports such as silica or magnesium chloride. In comparison to the silica-supported catalyst, the polymer particles described in catalyst preparation of the invention require no high temperature and prolonged dehydration steps prior to their use in catalyst synthesis, thereby simplifying the synthesis process and thus reducing the overall cost of catalyst preparation. Furthermore, the cost of the polymeric support used in the present invention is substantially cheaper than silica or magnesium chloride supports. In addition, the catalyst in the present invention uses significantly lower levels of catalyst precursors for catalyst preparation than silica or magnesium chloride supported catalysts. Also, the catalyst in the present invention is more active than conventional silica or magnesium supported Ziegler-Natta catalysts and some supported metallocene catalysts. It has been unexpectedly found that the catalyst compositions of the present invention has an activity of more than 60,000 g polyethylene per mmol of titanium per 100 psi per hour, thereby providing polymers of superior clarity having a melt flow ratio from 15 to 60.
According to one embodiment, a polyvinyl chloride support is used. The synthesis of the solid catalyst component in the present invention involves introducing the polymeric material described above into a vessel and then adding a diluent. Suitable diluents include isopentane, hexane, cyclohexane, heptane, isooctane and pentamethylehptane. The polymeric material is then treated with either a magnesium compound described above or a mix of a magnesium compound and aluminum compound of the type described above at a temperature in the range of 20xc2x0 C. to 150xc2x0 C., preferably 50xc2x0 C. to 110xc2x0 C. The ratio of organometallic compound to the polymer support can be in the range of 0.05 mmol-20 mmol per gram polymer, preferably, 0.1 mmol to 10 mmol per gram polymer, and more preferably 0.2 mmol to 2 mmol gram polymer.
The excess or unreacted magnesium compound or mix of the magnesium compound and the aluminum compound is removed by washing several times. Suitable solvents for washing purposes include iso-pentane, hexane, cyclohexane, heptane, isooctane and pentamethylheptane. The magnesium or magnesium-aluminum modified polymeric material is then treated with a transition metal compound of the type described above at a temperature in the range of 20xc2x0 C. to 150xc2x0 C., preferably 50xc2x0 C. to 110xc2x0 C. According to this invention, TiCl4, TiCl3, Ti(OC2H5)3Cl, VCl4, VOCl3, ZrCl4, ZrCl3(OC2H5) are preferred transition metal TiCl4 and ZrCl4 are more preferred. The produced solid catalyst component is then washed with a suitable solvent such as isopentane, hexane cyclohexane, heptane, isooctane and pentamethylheptane, preferably isopentane or hexane. The solid catalyst component is then dried using a nitrogen purge at a temperature in the range of 20xc2x0 C. to 100xc2x0 C., preferably 30xc2x0 C. to 80xc2x0 C.
The catalyst composition of this invention is not subjected to halogenation, e.g., chlorination treatments. The thus-formed catalyst component is activated with suitable activators, also known as co-catalysts or catalyst promoters. The preferred compounds for activation of the solid catalyst component are organoaluminum compounds.
The organoaluminum compounds which can be used in the present invention along with the solid catalyst component are represented by the general formulas R5nAIX3xe2x88x92n or R6R7Alxe2x80x94Oxe2x80x94AlR8R9, where R5, R6, R7, R8 and R9 each represent a hydrocarbon group having 1 to 10 carbon atoms; X represents a halogen atom and n represents a number satisfying 0xe2x89xa6nxe2x89xa63. Illustrative but not limiting examples of organoaluminum compounds include triethylaluminum, triisobutylaluminum, trihexylaluminum, diethylaluminum chloride, methylalumoxane, ethylalumoxane, and mixtures thereof. The organoaluminum compound in this invention can be used in the range of 1 to 1500 moles per one mole of transition metal in the said catalyst, and more preferably in the range of 50 to 800 moles per one mole of transition metal.
The catalyst described in the present invention can operate in polymerizing alphaolefins in solution, slurry and gas phase processes. A pressure in the range of 5 to 40 bars is suitable for the polymerization, more preferably 15 to 30 bars. Suitable polymerization temperatures are in the range of 30xc2x0 C. to 110xc2x0 C., preferably 50xc2x0 C. to 95xc2x0 C. In addition to polyethylene homopolymer, ethylene copolymers with C3-C10 alpha-olefins are readily prepared by the present invention. Particular examples include ethylene/propylene, ethylene/1-hexene, ethylene/1-butene and ethylene/1-octene. The molecular weight of the polymer can be effectively controlled by varying process conditions such as the hydrogen pressure used, as evidenced by the change in the melt index of the polymer produced. The catalyst compositions of the present invention are useful for olefin polymerization in the absence of electron donor compounds which are sometimes utilized to control the stereoselectivity of the catalyst during polymerization.